A full-color image forming apparatus of an electrophotographic process type using an intermediary transfer belt has been conventionally known. Specifically, in the image forming apparatus, toner images on a photosensitive member obtained by development by a developing unit are once primary-transferred onto the intermediary transfer belt and then four color toner images on the intermediary transfer belt are collectively secondary-transferred onto a transfer material.
The intermediary transfer belt includes a plurality of rollers inside the belt and is in a state in which the belt is stretched by these rollers. One of the rollers is a driving roller. The driving roller is rotationally driven for rotating the belt. A frictional force acts between the driving roller and the belt and by this force; the belt is conveyed by following the driving roller. Further, at a primary transfer position where the photosensitive member and the intermediary transfer belt contact each other, a primary transfer member is provided inside the belt. This primary transfer member is urged toward the photosensitive member while sandwiching the intermediary transfer belt therebetween.
Further, in order to meet diversification of users in recent years, a speed of a secondary transfer step or a fixing step is changed depending on the type of paper which is the transfer material. For example, when thick paper or an OHP sheet is used as a transfer material P, it has been known that a process speed of the secondary transfer step and the fixing step is lowered to about ½ of that when plain paper is used. This causes, in the case where the toner is secondary-transferred onto the transfer material P such as thick paper, improper transfer since an electric field becomes small compared with the plain paper. Further, in the case of the fixing, a heat conduction manner is weaker than that for the plain paper and therefore improper fixing occurs. For that reason, this problem is addressed by lowering the speed thereby to prolong a nip passing time.
As one of methods of changing this speed during the fixing, a method in which operations until the primary transfer are carried out at a predetermined process speed and after all the toner images obtained on the photosensitive member by development are transferred onto the intermediary transfer belt, the process speed is switched and then the secondary transfer step and the fixing step are performed has been known. When the process speed is changed, speeds of the photosensitive member, the intermediary transfer belt and the fixing device are changed. In this case, in a constitution in which a distance between the primary transfer position and a secondary transfer position is shorter than an image size, when all the toner images are completely primary-transferred, a leading end of the toner images has been in a state in which it passes through a secondary transfer portion. For that reason, e.g., in Japanese Laid-Open Patent Application Hei 07-225520, after the primary transfer is completely ended, the speeds of the intermediary transfer belt and the fixing device are switched and then an operation for idling the intermediary transfer belt one full circumference in a state in which the toner images are held on the intermediary transfer belt is performed.
However, in the image forming apparatus as described above, when the speed of the intermediary transfer belt is switched during image formation, due to differences of a gear train and motors (driving source), a difference in speed between the photosensitive member and the intermediary transfer belt can occur. Further, also during the switching, there is a potential difference between the member and the intermediary transfer belt and by the potential difference, an attraction force acts between the member and the intermediary transfer belt.
For example, in the image forming apparatus with a constitution as shown in FIG. 7, when the speed of a photosensitive member is first decreased, an intermediary transfer belt 16 is braked by the attraction force between the member 1 and the intermediary transfer belt 16. By this, a torque which is more than a drivable level is generated, so that a driving roller 16a and the intermediary transfer belt 16 cause a slip therebetween. Further, on the other hand, the intermediary transfer belt 16 is first decreased in speed, the photosensitive member 1 pulls the intermediary transfer belt 16. At this time, the speed of the intermediary transfer belt 16 is faster than a feeding speed of the driving roller 16a and as a result, the driving roller 16a and the intermediary transfer belt 16 slip. When this slip occurs in a short time, a phenomenon that a position of the formed toner images and a position of the paper are not aligned with each other occurs. Further, once the slip is caused, a frictional force between the driving roller 16a and the intermediary transfer belt 16 is lowered and the driving roller 16a continuously slips at it is, so that the driving roller 16a was in a state, in some cases, in which it cannot normally rotate the intermediary transfer belt 16.